1. Technical Field
The present invention relates to a radio frequency suppressing cable for suppressing the unwanted emission of radio frequency signals. Such a cable may be used for interconnecting devices and/or equipment such as may be used for radio frequency test and measurement purposes.
2. Description of the Related Art
In many equipments and fixed and portable installations there is a requirement to interconnect circuit boards, devices and accessories with flexible conductive links. However in order to comply with regulations relating to radio frequency emissions, it is desired to suppress the leakage of radio frequency radiation from these flexible conductive links. One well known technique is to use coaxial cables in which a conductor is insulated from and surrounded by a tubular, woven metallic screening conductor which in operation is usually earthed. The degree of flexibility of many co-axial cables is limited thus making them suitable for use in fixedly located equipments and static applications, such as TV antenna leads. The woven metallic screening conductor has the drawback that it allows spurious currents to flow down the outside of the cable. In certain applications standing waves have been reported as being supported on cables used in personal applications and this has lead to speculation of high specific absorption rate (SAR) due to coupling between these standing waves and the user.
In another known technique for reducing unwanted radio signal propagation, a ferrite bead is wrapped around a cable at a location which is as close as possible to the point of attachment of the cable to the equipment generating radio frequency currents. A drawback to using a ferrite bead or a plurality of such beads is that it or they are rigid thereby reducing the flexibility of the cable and also the radiation is suppressed only in the vicinity of the ferrite beads and not between them.
An object of the present invention is to provide radio frequency suppression substantially along the entire length of a cable.
According to the present invention there is provided a cable comprising at least one conductor and a resistive layer surrounding and being insulated from the at least one conductor, wherein the bulk resistance of material comprising the resistive layer is greater than that of the material comprising the at least one conductor.
In an embodiment of the present invention the thickness of the resistive layer is greater than the skin depth xcex4, the skin depth xcex4 being equal to   δ  =      1                  π        ⁢                  xe2x80x83                ⁢        σ        ⁢                  xe2x80x83                ⁢        f        ⁢                  xe2x80x83                ⁢                  μ          r                ⁢                  μ          o                    
where "sgr" is the conductivity of the material,
f is the frequency,
xcexcr is the magnetic permeability relative to that of free space, and
xcexco is the magnetic permeability of free space.
A cable made in accordance with the present invention provides continuous radio frequency suppression along its length. Depending on the number and size of the conductors in the cable it may be relatively thin and flexible so that it can be used with portable equipment and accessories or less flexible so that it can be used to interconnect fixedly mounted installations. The provision of the resistive layer serves to suppress any standing waves which may otherwise be present.
The thickness of the resistive layer may be between 2 and 10 times the skin depth.
The resistive material may comprise a carbon based material such as graphite, woven carbon fibre made from a graphite filament or graphite loaded plastics.